The present invention concerns a bottom insert for a mold cavity for the production of moldings of plastic material comprising a main body with a hot side which has a recess for receiving molten plastic material under pressure and a cold side having a recess, the inside contour of which is intended to come into contact with the molding to be produced and to shape the outside contour thereof, and a gate which connects the recess of the hot side to the recess of the cold side and is intended to introduce molten plastic material from the hot side into the cold side.
A mold cavity having such a bottom insert is known for example from DE 100 24 625 and is shown in FIG. 1.
The mold cavity is essentially used in the injection molding process. In injection molding the molding material which is in powder form or granulated is plasticised for example in a screw injection molding machine and then urged for example by axial displacement of the screw through the injection passage into the closed, generally cooled tool, for example a mold cavity, as is shown in FIG. 1.
When the mold or the molding cavity provided therein has been filled with the molten material the latter hardens due to cooling. Finally the tool or the mold cavity is opened and the finished molding is removed from the mold and ejected. The tool can be closed again and a fresh working cycle can begin with the renewed injection operation.
In addition, by means of injection molding, it is possible to produce hollow bodies which are inflated for example to form bottles or canisters in a subsequent working step. Those hollow bodies are also referred to as blanks or preforms. The mold cavity which is shown in FIG. 1 and which is known from the state of the art is intended for the production of such preforms. The mold cavity here comprises the actual cavity 1, the core 2, the neck ring 4, the support ring 5 and the bottom insert 6. In the assembled condition shown in FIG. 1 the mold cavity is enclosed or formed by those parts. The multi-part structure of the mold cavity serves inter alia for easy removability of the finished workpiece from the mold and for simple manufacture of the mold cavity itself. The plastic molding material is plasticised and homogenized in a suitable plasticising apparatus (not shown) and passed into the molding space 3 through the opening 8. After cooling of the molding material the molding can be removed from the mold and, in a further working step, held at the screwthread 13 or at the transport ring beneath the screwthread and inflated to form a bottle or a canister.
To achieve as rapid cooling as possible of the molding material and thereby to shorten the cycle time the mold cavity is generally cooled. For that purpose for example peripherally extending cooling grooves 12 are disposed externally on the cavity structure 1. In operation therefore the cavity structure is coaxially surrounded by a further tool portion or a plurality of tool portions so that the cooling grooves 12 form cooling passages with the surrounding tool portion. Disposed within the core 2 is a feed passage 7 through which cooling water can be passed into the core, the water flowing within the passage from left to right in FIG. 1 and then being passed back from right to left between the wall of the passage 7 and the inside wall of the core 2. As can be seen from the drawing the molding space 3 narrows in the proximity of the bottom insert 6. The narrowing contour of the molding space which is formed by the bottom insert 6 is denoted by reference 14 in FIG. 1.
To achieve effective cooling here the bottom insert 6 has a peripherally extending cooling groove 9. The cavity structure 1 has an inlet opening 10 and an outlet opening 11 for water cooling of the bottom insert 6.
For the most part the feed of the molten plastic material is effected by way of a nozzle which is actuated with a needle valve and which extends through the opening 8 of the bottom insert into the recess of the hot side and the outlet of which terminates immediately before the opening or gate of the bottom insert. The bottom insert represents the interface between what is referred to as the hot side of the overall tool and what is referred to as the cold side of the overall tool so that the bottom insert itself has a hot side which is shown at the right in FIG. 1 and a cold side which is shown at the left in FIG. 1. The molten plastic material is prepared under high pressure in the hot side of the tool and the bottom insert. Here it is essential that the molten material including the needle valve are at a working temperature of between about 280 and 285° C. When the needle valve is opened the molten plastic material is introduced into the molding space 3 through the nozzle. The molding space 3 and all surfaces adjoining same, that is to say also the inside wall 14 of the bottom insert, must be cooled as well as possible so that the molten material can harden very quickly and the preform can be removed. The bottom insert 6 must therefore be extremely well cooled on the one hand but on the other hand at its hot side it may not adversely affect the function of the needle valve, that is to say the needle valve must remain at the working temperature. The consequence of this is that the nozzle tip requires an additional heater so that both a heater and also cooling passages which in general are used simultaneously are arranged within the bottom insert 6. That means that rapid cooling of the preform in the mold cavity is only limitedly possible.
DE 100 37 739 proposes arranging a heat barrier element of heat-insulating plastic material between the nozzle and the bottom insert. That can admittedly reduce the heat flow between the nozzle tip on the one hand and the cold side on the other hand, but it makes little change to a considerable temperature gradient being formed within the bottom insert and therefore a large amount of heat flowing from the hot side to the cold side, which heat has to be dissipated by an additional cooling capacity.